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What is Differential Item Functioning (DIF)?. Answers the question, “Do item response curves differ significantly for subgroups of a population?” (e.g., gender, language, race, etc.) Often used to provide evidence of item bias in testing.

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What is differential item functioning dif l.jpg
What is Differential Item Functioning (DIF)?

  • Answers the question, “Do item response curves differ significantly for subgroups of a population?” (e.g., gender, language, race, etc.)

  • Often used to provide evidence of item bias in testing.

    • In most testing programs items that demonstrate DIF are routinely excluded from the test or not scored.

  • DIF is necessary but NOT necessarily a sufficient condition to detect item bias (i.e, an item can show DIF and NOT be bias).

    • For example, a poorly translated item can show DIF, which does not necessarily mean the item is bias.

  • With regard to polytomously scored items (items with greater than one possible point), DIF also serves the purpose of identifying non-scoring items.


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What is item bias?

  • Each group has its own set of cultural characteristics consisting of values, norms, attitudes, expectancies, etc…

    • …That sometimes reveal themselves statistically in test items.

  • Tests developed and tested in one population may show evidence of bias when used in another population.

    • Bias can occur through cross-cultural differences in the interpretations of the meaning of concepts and of items used to measure constructs.


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Item Characteristic Curves: Examples of DIF

  • The red and blue groups score differently on this item; a test candidate in the red group has a higher probability of answering this item correct.


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Identifying Non-scoring Items

  • 2004Q2: DIF analysis identified 8 items in 6 different languages that had never scored


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